Localized heavy showers affected
parts of south-eastern England during the afternoon and evening of 1st
August 1998. South-east London, Surrey and north-western parts of Kent
were worst affected by the storms, which produced, amongst other things,
pea sized hail and a tornado in the Guildford area. The storms also produced
some locally heavy downpours; weather observers across south-eastern England
reported maximum rainfall totals between 45 and 68mm and flash floods more
than half a metre deep were recorded in some places. Beckenham recorded
the most rain from these storms - here 68mm rain fell, including a fall
of 50mm in just one hour!

There was nothing particularly
unusual about these storms (unless of course you were in some way affected
by them in which case you'll have every reason to feel differently!) -
heavy localized downpours are common in north-western Europe during the
summer. Inevitably they will sometimes produce some hail and even tornadoes,
but even this isn't as unusual as one might thing, after all the UK averages
33 (mostly weak) tornadoes per year. However, I did remember to save some
interesting weather charts from this storm.

The surface synoptic chart
on the left shows the synoptic situation at 1800 GMT on 1st August. Britain
is sandwiched between an area of high pressure just west of Iereland and
low pressure and its associated frontal system over central Europe, whilst
the air over Britain was unstable enough to cause the storms in question.
The satellite picture (centre) shows the view from space at 1800 GMT. The
most striking feature is the mass of cloud over Germany, the Low Countries
and France whilst over southern England there's also some cloud, which
are the storms in question. The chart on the right hand side, meanwhile,
shows lightning strikes on 1st August. As can be seen,
there's a lot of them over southern England, most of them occuring during
the afternoon and evening.

The
diagram to the left is a tephigram (simply a graph showing air-temperature
and dewpoint plotted against pressure - the information comes from a radiosonde
[or weather balloon] ascent - used to help meteorologists make weather
forecasts) for Herstmonceaux on the south coast of England plotted from
data obtained during the early afternoon of the 1st August. The tephigram
shows the state of the atmosphere over south-eastern England close to the
time of the storms in the area where the storms occured. There's a few
interesting things to point out from the tephigram.

The wind arrows on the right
of the tephigram show that in the lower atmosphere (i.e. under 500mb) there
wasn't much wind. These sort of storms are steered by winds at around the
700mb level, so because there wasn't much wind at this level the
storms will have been moving very slowly which increases the chance of
a large amount of rain falling at any one place affected by the storm and
also means that any place affected by the storm will have been affected
by the storm for a longer period than if the storm had been moving a bit
quicker.

Of course the fact that the
storms were slow moving increases the chance of flooding and flash flooding
at any one place too, as does the fact that the tephigram shows that the
atmosphere was saturated between 925 and 700mb - the more saturated the
air in this region, the more rain can fall out of the storm cloud. The
winds in the higher levels of the atmosphere (between 300 and 250 mb) were
quite strong which means that the anvil at the top of the storm will have
spread out which would make the storm's cloud area much larger than it's
rainfall area - had these winds been lighter there would have been a smaller
horizontal spread of higher level cloud associated with the storm. In other
words the storm looked a lot bigger than it really was from the satellite
image!

The temperature curve (the
solid black line) shows that the freezing level in the storm was low -
at around 925 mb - and research has shown that a low freezing level encourages
hail to form.

The last thing to point out
here is that the area between the temperature curve and the red line to
its right is proportional to how active a storm will be. The greater this
area (known as CAPE), the more active the storm. Also, a large CAPE area
supports a strong updraft in the storm which in turn helps large hail to
develop.

Final RemarksSo, there you have it - the
profile of the atmosphere over south-eastern England made during the early
afternoon of 1st August 1998 shows that the atmosphere was ripe for some
rather nasty thunderstorms to develop. If you want to find out more about
thunderstorms you might like to visit the TORRO
website and the Dutch Stormchasing
website.